Determine the Maximum Tensile and Compressive Stresses in the Beam

[Northbysouth]

Homework Statement

Two vertical forces are applied to a beam of the cross section shown. Determine the maximum tensile and compressive stresses in portion BC of the beam

I have attached an image of the question

Homework Equations

σ = P/A +/- My/I_x

I_x = I_x + Ad² ---> parallel axis theorem

The Attempt at a Solution

I calculated the centroid to be:

x = 4.5 in

y = 3 in

I did this by dividing the shape into two rectangles, determining the centers of those rectangles from an arbitrarily assigned origin, O, which I assigned to be at the bottom left of the lower rectangle.

I then used the parallel axis theorem to calculate the Inertia, I_x

P = 0 because the sum of the forces is 0.

M is the greatest moment value, which I found to be 600 kip-in.

My confusion is how do I determine the y value? I believe that the greatest tensile and compressive stresses would be at the bottom and top of the bar respectively, but my notes indicate that the correct y-values are 3 in for the tensile stress and 5 in for the compressive stress. I had thought that the y values was from the centroid of the cross-section which makes me think it should be 3 in for the max compressive and tensile forces.

Can anyone clarify this for me?

I know the final answers are

8.82 ksi T

14.72 ksi C

 

[Northbysouth]

Never mind, I figured it out.